Here's my favorites:
Monsternomicon: This is EXACTLY how all monster books should be. Lots of information and excellent monsters.
Monster Manual: Let's face it, even if there weren't any other monster books you'd still be set with this book.
Savage Species: The Templates, PrC's, and Monster Classes made this an instantly used book at my gaming table.
Denizens of Darkness: While I don't really like Ravenloft, this book has some great, dark creatures that are easily usable in any setting.
Fiend Folio: Thank God this book wasn't as goofy as the original. As it stands it's a must have for planar campaigns.
Kane